The nature and quantitative aspects of epithelial cell adhesion are reviewed, and their relevance discussed in relation to the homeostasis of the dento-epithelial junction. The design, construction and testing of a miniaturised parallel-plate shearing apparatus for the measurement of cell adhesion based on the radial flow chamber principle of Fowler & McKay (1980) is described. Cultures of an established epithelial cell line on glass coverslips were exposed to flow conditions for varying times in the shearing chamber at 37°, 8° and 4°C, and subsequently photographed under standardized conditions. The critical shear radius (CSR) was determined by densitometry from a negative film and the minimum distraction force (MDF) at the CSR calculated using the measured flow rate and predetermined viscosity values of the medium. The calculated mean MDF values at 37°C ranged from 1.04 to 1.36 pascals, and was independent of the culture inoculation density (2.5 x 10
5 to 10
6 cells ml-1) and thetime (range: 5 to 20 min) of exposure to shearing conditions. Cell-to-cell adhesion in multilayer cultures was assessed by a cell-separation index (O) which represented the proportion of cells detached in a specific annulus per unit shearing force. The minimal force necessary to separate the upper layer(s) of cells was calculated to be significantly less than cell-to-substratum adhesion (MDF) being in the range of 0.43 to 0.64 Pa. Measurements of cell-to-substratum adhesion at 4°C demonstrated a three to four fold increase of the MDF (6.19 Pa) compared to that at 37°C (1.35 Pa). Part of this adhesion was resistant to mild trypsinisation and was stabilised by low temperature, and by treatment with concanavalin A or colchicine. A classification of cellular adhesion as trypsin-sensitive (TSA) and trypsin-resistant adhesion (TRA) on the basis of the different temperature and protease susceptibility is proposed with the corresponding physiological functions of "frictional" and "tractional" adhesion respectively. The implications of the dual adhesion hypothesis are discussed with respect to the integrity of the dento-gingival junction
